JPH0480349A - High strength galvannealed steel sheet having baking hardenability and excellent in powdering resistance and its manufacture - Google Patents

Abstract

PURPOSE:To manufacture a galvannealed steel sheet excellent in powdering resistance by applying a galvannealing layer having the specified concn. of Fe to be incorporated to a steel sheet having a specified compsn. in which each content of Nb, B and Ti is prescribed. CONSTITUTION:A galvannealing layer having 5 to <15% Fe concn. is applied to a steel sheet contg., by weight, <=0.010% C, <=0.5% Si, 0.15 to 0.80% Mn, <=0.030% P, <=0.03% S, <=0.100% Al, <=0.0050% N as well as >=2Al/N, 0.0005 to O.0050% B, 2XC(%) to 7.5XC(%) Nb, 0.003 to 0.030% Ti and the balance iron with inevitable impurities. In this way, the galvannealed steel sheet in which the peeling of the galvannealing layer is prevented and having improved baking hardenability and high strength can be obtd.

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention relates to a bake-hardenable high-strength galvanized steel sheet for press working that has excellent powdering resistance and a method for producing the same. ru m is 40mg/rr?
This invention relates to a high-strength alloyed hot-dip galvanized steel sheet that does not peel off during plastic deformation during press working even when thickened (on one side).

(Conventional technology) In the automobile industry, high-strength cold-rolled steel sheets have been increasingly adopted, mainly for strength members, with the aim of reducing the weight of car bodies from the viewpoint of resource and energy conservation. High-strength steel sheets are now being actively applied to parts called so-called outer panels.In particular, for outer panels, with the aim of ensuring Punt resistance, The use of steel plates with hardening properties is increasing for paint baking, which increases strength through processing.In general, baking is required to increase the yield point by processing to 3 kg/114 or higher.

On the other hand, recently, the issue of extending the lifespan of automobile bodies has begun to attract attention, and the automobile industry is applying surface-treated steel sheets with thicker plating films to many parts, including inner and outer panels. Things are progressing. In particular, for exterior panels, the use of high-strength, surface-treated steel sheets for thick coatings with excellent hardening properties is being considered.

However, the thicker the film plated on the surface of the steel sheet, the more likely it will peel off during press working, which impairs the rust-preventing effect of the car body and worsens the working environment. Furthermore, this problem becomes even more apparent when a large amount of alloying elements are contained in order to improve strength, such as in high-strength steel sheets.

The peeling phenomenon of the plating film is called powdering property or flaking property, and as a means to solve these defective phenomena, Aρ is added to the hot-dip galvanizing bath as disclosed in (+) Japanese Patent Application Laid-Open No. 52131934. (2) A method of controlling the Fc concentration in the alloyed galvanized layer within a specific range as disclosed in JP-A No. 61270% No. 2, and methods mainly based on improving the plating operating conditions. has been considered so far. It is certainly possible to reduce the peeling of plating to some extent by adopting these methods, especially when the basis weight is large and
In the case of high-strength alloyed hot-dip galvanized steel sheets, the problem has yet to be solved.

(Problems to be Solved by the Invention) In view of the current situation, the present invention is capable of preventing peeling of the plating layer, which has been a problem due to an increase in the plating area weight and high strength using alloying elements. Another object of the present invention is to obtain a high-strength alloyed hot-dip galvanized steel sheet with excellent bake hardenability, and to provide a method for producing the same.

(Step F to solve the problem) In order to solve the above-mentioned problem, the present inventors have optimized the plating operating conditions and conducted various studies on the composition of steel sheets. The alloyed hot-dip galvanized steel sheet has very little peeling even when the amount is increased, has hardenability when baking over about 3 kg/m4, and has high strength over 35 kg/m4. Low carbon steel is used as base material, combined addition of Nb equivalent to C and B in a solid solution state, further addition of Ti to stably exist as solid solution B, 2) Fe concentration in the galvanized layer is increased from 5 to 5.
It has been newly discovered that the product is produced by the synergistic effect of alloyed galvanizing treatment with less than 15% zinc alloying.

The present invention was made based on this knowledge, and its gist is that C: 0.01 in weight%.
0% or less, Si: 0.5% or less, Mn: 0.15 to 0.80%, P: 0.030% or less, S: 0.03% or less, Ml: 2 or more in AIl/N 0.100 % or less, N:
0.0050% or less, B: 0.0005 to 0.0050%, Nb: 2
×C (%) ~ 7.5 × C (%), Ti: 0.003
~0.030%, with the balance consisting of iron and unavoidable impurities, and the Fe concentration in the galvanized layer is 5.
A bake-hardenable high-strength alloyed hot-dip galvanized steel sheet with excellent powdering resistance characterized by being coated with an alloyed galvanized layer of less than ~15%.

In addition, in weight %, C: 0.010% or less, Sf: 0.5% or more, Mn: 0. I5~0.80%, P: 0.030% or less, S: o, oa% or less, AΩ: l) /N from 2 to 0.100%, N:
0.0050% or less, B: 0.0005 to 0.0050%, Nb: 2x
C (%) ~ 7.5 x C (%), 〇 for T, 003 ~ 0.0
A steel billet containing 30% and the remainder consisting of iron and unavoidable impurities is hot rolled at a finishing temperature of 750°C or higher and a coiling temperature of 740°C or lower, and then cold rolled at a reduction rate of 60% or higher. After that, recrystallization annealing is performed on a continuous hot-dip galvanizing line, and after forced cooling, hot-dip galvanizing is performed at 420-500°C, followed by alloying treatment at 400-600°C to reduce the Felff degree in the plating layer. The present invention provides a method for manufacturing a bake-hardenable high-strength alloyed hot-dip galvanized steel sheet with excellent powdering resistance, characterized by applying an alloyed galvanized layer of less than 5 to 15%.

The present invention will be explained in detail below.

C is 0.010% to improve the press working properties such as r value and elongation of the steel plate, and baking is 0.010% to ensure hardenability.
The following shall apply. 0.00 to obtain particularly high press workability
It is preferably 5% or less.

Sl is effective in improving the strength of steel sheets, but on the other hand, when its content increases, it deteriorates the adhesion of plating, so 0.5
% or less. 0.10 if high strength is not required
% or less is preferable.

Mn is a component added to increase the strength of steel sheets, and is 0
．． 15% or more. On the other hand, if it is too large, the r value will deteriorate, so it should be set at 0.80% or less.

P is the most effective element for increasing strength, and conventionally it was contained in large amounts to increase strength, but it deteriorates the bond between the steel sheet and the plating layer and causes the plating film to peel off during press working, so it is .030% or less.

S is effective in generating fine sulfides and increasing strength, but if too much S deteriorates ductility, so 0.03
0% or less.

A and 9 are contained in order to fix N and precipitate fine AΩN to increase strength, as well as to effectively exhibit the effect of B described below.
/N is added in an amount of 2 or more, but too much will only increase the alloy cost, so it is limited to 0.10% or less.

N is a harmful component in the present invention, and if its content is too large, it becomes a factor that deteriorates the r value and ductility, and also inhibits the effect of containing B, so it is set to 0.0050% or less.

B has been added to raw materials to fix N, but
In the present invention, B is contained based on the new knowledge that solid solution B suppresses plating peeling. 0.0 to get that effect
The content must be 0.005% or more.

On the other hand, if the number of particles increases, the r value will decrease and the plating peeling prevention effect will be saturated, so the content should be 0.050% or less.

Nb fixes C in solid solution and requires a Nb/C (vt% ratio) of 2 or more in order to achieve both anti-aging property and bake hardenability. However, the content increases and Nb/C becomes 7
．． If it exceeds 5, the seizure hardening will be reduced, so it should be set to 7.5 or less.

Tj has the effect of preventing hot brittle cracking caused by S, which occurs in direct rolling, and also fixing N, ensuring a more stable solid solution state of B, and helping to prevent peeling of the plating.

To obtain these effects, the content must be 0.003% or more. On the other hand, if the content increases, the seizure hardening will decrease and the hot-dip plating properties will deteriorate, so the content should be 0.030% or less.

Molten steel having the above-mentioned chemical composition is melted in a converter or electric furnace, made into steel slabs by continuous casting or ingot-blending, and then hot-rolled. After continuous casting, the M-piece may be directly hot-rolled while still hot, or may be charged into a reheating furnace in the form of hot pieces and then hot-rolled. The heating temperature in the reheating furnace is 1000-1300°C. The finishing temperature during hot rolling is set at 750° C. or higher because lower temperatures result in surface defects called zinc. The upper limit of the finishing temperature does not need to be regulated from the viewpoint of workability, but from the viewpoint of reducing roll wear, it is set at 950.
℃ or less is preferable. The coiling temperature after finish rolling is 740℃ because the higher the temperature, the larger the crystal structure and the decrease in strength.
The following shall apply. Note that if the r value is important, it is preferable to wind the film at a high temperature of 700° C. or higher.

Next, in cold rolling, if the cold rolling rate is too low, the recrystallization temperature will decrease to TR.
However, it is difficult to secure a high r value, so rolling is performed at a rolling reduction of 60% or more. In order to improve press workability and obtain high strength, a rolling reduction ratio of 75 to 85% is preferable.

After cold rolling, hot-dip galvanizing and alloying treatment are performed on a continuous hot-dip plating line, but in order to improve the peeling resistance of the plating and to apply a plating that does not peel during press working, in addition to the above-mentioned component composition, These processing conditions are important. Masu,
It is preferable that the plate be passed through a non-oxidizing furnace and a reducing furnace to give recrystallization annealing workability by a normal method, or that the annealing temperature be 700° C. or higher. After recrystallization, it is forcibly cooled down to the temperature of the galvanizing bath using a cooling means such as a water cooler or gas jet, and then hot dip galvanized at 420 to 500°C. If the plating temperature is too low or too high, it will not be good for the plating properties and will deteriorate the woodwind property and corrosion resistance, so the above 420 to 50
The temperature shall be 0°C. In addition, AΩ is 0 in the plating bath.
．． Inclusion at a concentration of 0.08 to 0.30% is effective for controlling alloying.

Next, alloying treatment conditions are one of the important factors governing the removability of plating by press working, and
Alloying treatment is performed at a temperature of 0° C., and the FC concentration in the plating layer is controlled to be less than 5% to 15%.

As the alloying temperature decreases, the alloying reaction slows down.
If it is too high, alloying will progress too much and it will be difficult to keep the Fet8 degree in the plating layer to less than 5 to 15%, so the temperature is set at 400 to 600°C. If the Fc6 degree in the plating layer is too low, the corrosion resistance and weldability after painting will be poor, so it should be reduced to 5%.
On the other hand, if it is too large, peeling of the plating will become a problem, so it should be less than 15%.

Furthermore, after the alloyed galvanizing treatment, F
Applying an e-Zn-based upper layer plating is effective for further improving plating peeling resistance.

Further, after that, an elongation rate of 0.5% or more is applied using a skin pass or a tension leveler to eliminate the yield point elongation and correct the shape.

The present invention can be applied to galvanized steel sheets such as Zn-3n in addition to alloyed hot-dip galvanized steel sheets, and the effects of the present invention can be similarly enjoyed.

(Example) The steel shown in Table 1 was melted and made into a 4.0 mm hot rolled plate under the hot rolling conditions shown in the same table, and then cold rolled to 0.8 m +
m, then hot-dip galvanizing and alloying treatment were performed under the conditions shown in Table 2. The amount of zinc coating is BDg/d per side.
I made it thicker. The steel plate obtained after applying 1.0% skin pass was subjected to a tensile test, and the r value, BH property, and peeling of the plating layer due to processing were investigated. Regarding the releasability of the plating layer, drawing was performed using a square tube punch, cellophane tape was attached to the corners of the square tube, the tape was peeled off, and the amount of plating attached to the tape was measured. The spacing between the host rocks was classified into four levels: large, medium, small, and very small, and the peelability of the plating was evaluated by JF rating. Various measurement results are shown in Table 2.

As is clear from Table 2, samples A to E, which meet all the compositions, hot rolling conditions, hot-dip galvanizing conditions, and alloying conditions of the present invention, have excellent plating peeling resistance and bake hardenability. It is evaluated as a high-strength alloyed hot-dip galvanized steel sheet with a high r value. On the other hand, both samples F and H, which have a high content of P or Ti, have a very large amount of plating separation. Moreover, samples G and H, which have a large content of Nb and Ti, cannot obtain bake hardenability.

(Effects of the Invention) The alloyed hot-dip galvanized steel sheet according to the present invention has bake hardenability that ensures high strength and increases the strength by baking the paint, and also has sufficient deep drawability due to its high r value. In addition, even when subjected to severe processing, there is extremely little peeling of the plating layer, and this peeling resistance is sufficient even with thick plating, making it possible to obtain steel sheets with significantly superior performance than conventional steel sheets. . This high-strength alloyed hot-dip galvanized steel sheet is suitable as an abrasive for automatic use, which requires Punt resistance, severe workability, and a longer car body life than conventional ones.It also reduces the defective rate of molded products and improves the working environment. Brings extremely useful effects industrially.

Out wish Man new Day Wooden Iron Co., Ltd. sub-agent

Claims (2)

[Claims] 1. In weight%, C: 0.010% or less, Si: 0.5% or less, Mn: 0.15 to 0.80%, P: 0.030% or less, S: 0.03% or less, Al: Al /N is 2 or more and 0.100% or less, N: 0.
0050% or less, B: 0.0005 to 0.0050%, Nb: 2×C (%) to 7.5×C (%), Ti: 0.003 to 0.030%, and the balance is iron. Bake-hardenable, high-strength alloyed melt with excellent powdering resistance, characterized by applying an alloyed galvanized layer with Fe concentration in the galvanized layer of 5 to less than 15% to a steel sheet consisting of unavoidable impurities. Galvanized steel. 2. In weight%, C: 0.010% or less, Si: 0.5% or less, Mn: 0.15 to 0.80%, P: 0.030% or less, S: 0.03% or less, Al: Al /N is 2 or more and 0.100% or less, N: 0.0
050% or less, B: 0.0005 to 0.0050%, Nb: 2×C (%) to 7.5×C (%), Ti: 0.003 to 0.030%, and the balance is iron. After hot rolling a steel billet containing unavoidable impurities at a finishing temperature of 750°C or higher and a coiling temperature of 740°C or lower, and cold rolling at a reduction rate of 60% or higher, a continuous hot-dip galvanizing line is used. After recrystallization annealing and forced cooling, hot-dip galvanizing is performed at 420 to 500°C, followed by alloying treatment at 400 to 600°C, resulting in alloying in which the Fe concentration in the galvanized layer is less than 5 to 15%. A method for producing a bake-hardenable high-strength alloyed hot-dip galvanized steel sheet with excellent powdering resistance, characterized by applying a galvanized layer.